11.0.4 11.6: Flatworms, roundworms, and segmented worms come in all shapes and sizes.

It’s a good thing that biologists don’t run bait shops—if they did, a simple trip to buy worms for fishing would be way too complicated. The name “worm” is commonly applied to a long, skinny, slimy animal without a backbone, but you can find animals fitting that description in eight different phyla. You will never encounter most of them, and that’s a good thing, because many are parasites that cause really unpleasant diseases. We consider here just three phyla—flatworms (phylum Platyhelminthes), roundworms (phylum Nematoda), and segmented worms, or annelids (phylum Annelida)—that illustrate most of the diversity of the animals we call worms.

All of the worms described here have defined tissues and are protostomes—the gut develops from front to back. Unlike the cnidarians, however, the worms (along with all of the remaining animals we discuss in this chapter) develop with a body plan characterized by bilateral symmetry, which adapts them for forward movement.

It is important to note, however, that worms do not make up a monophyletic group. Their general body plan—that is, a long, cylindrical, legless, soft-bodied animal without a backbone—has evolved several times independently, and their evolutionary relationships are not always obvious. Before their evolutionary relationships were understood, these groups of organisms were all classified by Linnaeus as worms, simply because of these similarities in their appearance.

We know today, however, that convergent evolution is responsible for much of the resemblance and that the segmented worms (annelids), for example, are probably more closely related to mollusks (including snails, clams, and octopuses) than to the roundworms. And the roundworms are believed to be more closely related to the arthropods (insects) than to either the flatworms or the segmented worms (see Figure 11-3). From an evolutionary perspective, “worm” is now an obsolete—and potentially misleading—label.

Flatworms Unlike the roundworms and segmented worms, flatworms have no body cavity. That is, the space between the body wall and the digestive tract is not a fluid-filled cavity. Possibly as a consequence of this, the flatworms have no specialized respiratory or circulatory organs and so have a reduced capacity for diffusion of gases and nutrients.

Like the segmented worms, but unlike the roundworms, flatworms grow by adding body mass rather than by molting (FIGURE 11-10). Flatworms have well-defined head and tail regions, with clusters of light-sensitive cells called eyespots that help them orient themselves within their environment. Most flatworms are hermaphroditic, each individual producing both male and female gametes, and they engage in both sexual and asexual reproduction. Flatworms include more than 20,000 species and can be parasites (the flukes and the tapeworms) or brilliantly colored, free-living aquatic creatures.

Many flatworms have a digestive system, though among those that do, their gut has only one opening, requiring them to both consume food and eliminate undigested waste through the same opening. The digestive systems of roundworms and segmented worms, conversely, have two openings. One large group of flatworms that lack a digestive system is the tapeworms. These parasitic worms live in their host’s gut and absorb nutrients from the host directly through their body wall.

All 5,000 species of tapeworms are parasites, and most have a two-stage life cycle that is split between two different host species. For example, the common tapeworm that infects dogs spends the other half of its life cycle in fleas, while the human pork tapeworm splits its life cycle between pigs and humans. Tapeworms have long, flat bodies made up of repeated segments, each of which is a reproductive unit. Mature tapeworms spread by breaking off segments, which are then shed in the feces of an infected individual.

Roundworms The roundworms, also called nematodes, are probably the most numerically abundant animals on earth—a spoonful of garden soil contains several thousand individuals, and some species produce more than 200,000 eggs every day. Unlike segmented worms and flatworms, they grow by molting. More than 90,000 roundworm species have been named, and there may be five times as many species that have not yet been identified (FIGURE 11-11).

Many soil-dwelling roundworms live in the roots of plants, damaging or even killing the plant. About 15,000 species of roundworms are parasites of other animals, and roundworms are responsible for a large number of human diseases. Most roundworms are transmitted by fecal contamination of soil or food.

Tiny parasitic roundworms called filariae are responsible for several tropical diseases, including some that have an enormous social and economic impact in certain parts of the world. Elephantiasis, for example, is a disease in which filariae transmitted by the bite of a mosquito block the lymph ducts so that fluid accumulates in the limbs or scrotum, causing grotesque swelling. These filariae are found in India, Africa, South Asia, Pacifica, and tropical regions of the Americas.

Segmented Worms (Annelids) The segmented worms, or annelids, number about 13,000 species and are easy to recognize by the grooves running around the body that mark the divisions between segments—if you’ve seen an earthworm, you are familiar with these grooves. The segmented worms are protostomes with defined tissues, and they do not molt. They are organized into three different groups: marine polychaetes (pronounced pol-lee-keets), terrestrial oligochaetes, and leeches (FIGURE 11-12).

Polychaetes are marine worms, living on the seafloor. Polychaete means “many bristles,” and the combination of segments and bristles makes polychaetes easy to recognize. Some species burrow through the mud and extract organic material from it. Tube worms use sand grains or limestone to make a tube in which they live, with just their waving tentacles exposed. Small particles of food are trapped in mucus on the tentacles and transported to the worm’s mouth.

Earthworms, which belong to the group called oligochaetes (“few bristles”), are the annelid worms you are most likely to have seen. The night craphelaner is a typical earthworm. It gets its name from its habit of emerging from its burrow on rainy nights to craphelan across the surface of the ground. More than 4,000 earthworm species have been named, ranging in size from less than half an inch (about a centimeter) to the enormous length of the giant Gippsland earthworm, 7–10 feet (2–3 m)!

Earthworms are bulk-feeders—that is, as an earthworm burrows, it consumes particles of soil and organic material. The organic material is digested as it passes through the worm’s one-way gut, and the fecal material plus the inorganic part of the soil is excreted as feces, called castings. Earthworm castings are valued by gardeners as soil supplements, and an earthworm can produce its own weight in castings every day. Earthworms’ activities mix the soil components, create a more uniform mixture of nutrients, and expedite the breakdown of organic materials in the soil, thus making the nutrients available for plants. Because they perform all of these tasks, earthworms have enormous economic value to agriculture. A government study published in 2008 estimated the economic value of earthworms in Ireland alone to be more than $1 billion per year.

Have you ever waded in a pond or swamp and emerged to find a long, dark brown worm clinging to your leg? If so, you have met the third major group of annelids, the leeches. Probably you pulled the leech off (not an easy thing to do, because leeches are both slippery and stretchy) and threw it as far as you could. If you had looked at it carefully, though, you’d have noticed that, like earthworms, leeches have segmented bodies.

Not all leeches are blood-suckers—in fact, more than half the leech species are predators. The horse leech, for example, which reaches a length of 8 inches (about 20 cm), feeds on smaller annelid worms, snails, and aquatic insect larvae.